Vehicle safety device

ABSTRACT

An inflatable element in the form of a tension tube, and method of making same. The method involves forming a tube by a knitting process, then heatsetting the tube whilst simultaneously maintaining it under lengthwise tension. The tension in the tube is maintained during cooling of the tube. An option relaxation stage may also be included.

[0001] This invention relates to a vehicle safety device and to a methodof manufacturing such a device.

[0002] The present invention is concerned particularly with safetydevices of the type described in U.S. Pat. No. 5,322,322. This patentdiscloses a safety device for fitting in a vehicle side window andwhich, when inflated, will stretch across the window to prevent theoccupant passing through the window in the event of an accident. Thedescribed device comprises an inflatable element in the form of abraided tube, which is attached at spaced points on opposite sides ofthe vehicle window and is normally located out of sight in the windowframe. Upon detection of a side impact, the braided tube is inflated,causing its diameter to increase, and its length to decrease. As aresult it becomes stretched between its mounting points and these arepositioned so that the inflated tube stretches taut across the window ina semi-rigid state to protect the occupant in the manner described.

[0003] The present invention is directed towards the inflatable elementwhich is used in devices such as described in U.S. Pat. No. 5,322,322.Although the primary intended use for such elements is in vehicle safetydevices, particularly for side impact protection, it is not intendedthat the inflatable elements of the present invention be limited to suchuse.

[0004] In general terms such inflatable elements comprise a tubularstructure made from yarn and means for inflating the tubular structuresuch that it simultaneously increases in diameter and decreases inlength. The decrease in length is constrained so as to generate atension or load which renders the element semi-rigid, once inflated. Theinflating means may comprise a web of flexible material which is formedby impregnating the tubular structure with resin. Alternatively, aseparate impermeable bag or bladder may be inserted within the tubularstructure for the same purpose.

[0005] One problem with the braided tubes described in U.S. Pat. No.5,322,322 is that they are effectively limited to single tube systemsbecause it is not yet possible to braid a double or triple tube or toform integral webs attached to the tube.

[0006] Another difficulty is that it is not possible to form discreteopenings in the tube, other than by forcing a hole between the threadsof the braid. Such openings might, for example, be necessary in order toenable insertion of a bladder if the ends of the tube are closed, or toenable a bladder fixing strap to be taken out through the tube wall toprevent it moving, or to hold it in a fixed position.

[0007] As a general matter, the braid structure is very loose andsignificant openings can easily be made, either accidentally ordeliberately, by slightly working the yarns apart. This can in turn leadto problems with the bladder protruding through the braid in the mannerof an aneurysm.

[0008] Such difficulties can be overcome by manufacturing the inflatableelement by a knitting process.

[0009] British Patent No. 2339557 describes an inflatable element forside impact safety devices which comprises a knitted tubular structurehaving a diamond pattern of knitting which attempts to emulate thebraided structure described in U.S. Pat. No. 5,322,322. It is not clear,however, how such a structure generates load on inflation to become asemi-rigid structural member, which is what is required to form aneffective barrier across the window in an impact.

[0010] The present invention seeks to provide an inflatable elementwhich performs to the requirements of a tension generating tubularassembly.

[0011] According to a first aspect of the invention there is provided aninflatable element comprising a tubular body of knitted fabric and meansfor inflating the tubular body, said tubular body being formed from atube made by knitting using synthetic yarn, followed by a tensioning andsetting operation intended to improve the strength and tensiongenerating performance of the knitted tube during inflation.

[0012] According to a second aspect of the invention there is provided amethod of making an inflatable element comprising a tubular body ofknitted fabric and means for inflating the tubular body, said methodcomprising forming a tube by a knitting process using synthetic yarn,thence subjecting the knitted tube to tension and setting processes toimprove the strength and tension generating performance of the knittedtube when inflated.

[0013] The inflatable element of the invention preferably takes the formof a tension tube, by which is meant a tubular assembly which, uponinflation, will be subject to an increase in diameter and a decrease inlength such that, if the length is constrained, then the tube willgenerate tension or load between the constrained ends. This tension orload is necessary for the tube to provide the necessary semi-rigidstructural member in its application as a vehicle side impact safetydevice.

[0014] A tension tube made simply by a knitting process, such asdescribed in GB 2339557, will generate a degree of tension between itsconstrained ends on inflation; however, the reduction in length anddevelopment of load are not of sufficient magnitude for use incommercial applications. To produce a knitted structure with thecharacteristics required for use in commercial applications, the knittedtube is, in accordance with the invention, subject to a tensioning andsetting process to ensure that the yarns making up the knitted structureare set in a geometric configuration which is best suited to thegeneration of tension during inflation. A preferred knitting patternwould be 2:1 Tricot and 1:1 Tricot; however, other knitting patternscould be used to produce a similar effect.

[0015] The yarn used to fabricate the tube is synthetic yarn, forexample polyester, and is preferably of the multifilament type. However,yarns of other synthetic materials could be used, provided that the heatsetting conditions are varied appropriately. Examples are nylon,polypropylene, viscose and HMWHDPE.

[0016] A tension tube, whether it be braided or knitted is made up froma multiplicity of yarns which are in such a form that, as the tubeincreases in diameter during inflation, then the angle of the yarnswithin the structure changes. As the angle increases relative to thelongitudinal axis of the tube then the tube will reduce in length. Ifhowever the tube is held at each end so that its length cannot reduce,then the inflation forces the tube to tighten, thus generating tensionwithin the tube because the tube is not permitted to shorten, as itwould like.

[0017] It is believed that the angular change of the yarns will be mosteffective in generating tension until they reach an angle of about 45°.

[0018] The angles of the yarns of a typical example of a knitted and settube are illustrated in FIG. 2 which is orientated so that the verticalaxis of the drawing is parallel with the longitudinal axis of the tube.As can be seen, the starting angle might be around 10° with a stitchlength of 6.62 mm. The distance between the rows of stitches in themachine direction will be approximately 6.5 mm; the startingcircumference is approximately 104 mm. During inflation thecircumference increases; as the circumference is increased to 385 mm theangle of the yarn will now be around 43° and the distance between therows of stitches will have reduced to approximately 4.9 mm. Since thetube is manufactured from synthetic yarns which have an elastic modulus,the whole tube will also want to extend under the aforesaid tension asthe diameter increases.

[0019] In a particular embodiment of the invention, the tube is knittedon a Raschel double needlebed knitting machine using 1100 DTEX polyestermultifilament yarn. The machine used is 9 gauge and the tube is knittedusing 44 needles on each bed. The fabric construction is 2:1 Tricot and1:1 Tricot with a stitch length of 6.75 mm.

[0020] The knitted tube is next subjected to a heating and settingoperation by being passed over heated rollers whilst beingsimultaneously held lengthwise under tension. The tube is allowed tocool whilst maintaining the tube under tension. After cooling, thetension is released. If a low residual heat shrinkage at elevatedtemperatures is required, then a relaxation stage may be included in thesetting operation.

[0021] When the tube comes off the knitting machine it has a fairlyunstable structure composed of rows of loose stitches, the legs of whichare significantly curved. The subsequent setting operation has theeffect of compacting the structure, straightening the legs and aligningthem in predominantly the machine direction—i.e. along the longitudinalaxis of the tube. The knitted tube has a significant amount of slack inthe system due to the nature of the knitting process and the settingprocess is vital in minimising this slack because it has the effect ofpermanently aligning the yarns in the optimum direction.

[0022] The finished tube is cut to a length determined by the particularapplication to which it is to be put. For example, for the applicationin a side impact vehicle safety device, the length is dictated by thedimensions of the window in which it is to be fitted. Typical lengthsfor this purpose are in the range 1300 mm to 2000 mm. Cutting of thetube can be achieved in various ways, but preferably the cut ends aresealed to prevent fraying.

[0023] For example a heated knife can be used to both cut the tube intolengths and to seal the cut ends. Preferably the tube is left open at atleast one end to enable insertion of a bladder. If both ends are leftclosed however, the bladder can be inserted through a hole or holesspecially knitted in the tube for the purpose.

[0024] The temperature to which the tube is raised during the settingoperation, and the tension applied will depend upon the materials usedand the ultimate properties required. Typically for polyester yarns, theheated rollers can be at a temperature of 230° C. and the tube can beheld at a tension of 100 Kgs.

[0025] During the setting operation the individual yarns within theknitted tube become set in the alignment most suited for the tube toreduce in length and, if constrained, to generate tension duringinflation.

EXAMPLE

[0026] A tension tube of 1710 mm length manufactured by theabove-described method had a starting diameter of 32 mm and wasconstrained at its ends at a fixing pitch of 1595 mm. An inflatablebladder incorporated in the tube was then inflated to a pressure of 2.2Bar and a diameter of 120 mm. Under these conditions a tension of 500Kgs was generated between the fixed ends of the tube.

[0027] A feature of the knitting process is that multiple tubes andcomplex tubular structures can be knitted which have discrete openingsand areas for inflation. Furthermore the tubular structure can beknitted together in specific areas to produce single fabric if localisedstrengthening or seaming is required.

[0028] In an embodiment of the invention, the inflated shape of theinflatable element is varied by selectively increasing or reducing thestitch length within the knitted tube so that the effective diameter ofthe inflatable element will either be increased or decreased in the areawhere the stitch length has been varied.

[0029] It will also be apparent that, for a given number of knittingneedles in a given structure, by modifying the stitch length theeffective diameter can be varied as described above and, in addition,the tension generating capability will be altered. In general increasingthe stitch length will increase the effective diameter and also the loadgenerating capability at a given inflation pressure.

[0030] In order that the invention may be better understood, anembodiment thereof will now be described, by way of example only andwith reference to the accompanying drawings in which:

[0031]FIG. 1 is a schematic view of an apparatus suitable for carryingout the method of the invention; and

[0032]FIG. 2 is a diagram intended to assist the explanation of thevariation of yarn angle during inflation.

[0033] Referring to FIG. 1 there is shown one example of heat settingequipment for providing the setting operation on the knitted tube. It isassumed that the knitted tube has already been manufactured in themanner already described and is rolled up on a roller 1; however, theprocess could of course be continuous.

[0034] From the roller 1, or other let-off mechanism, the knitted tubeis fed at a controlled pre-tension to a set 2 of driven rollers runningat a fixed speed, typically 400 mm/min. The tube then passes round aguide roller 3 and then round two heated rollers 4 arranged so thatfirst one surface, then the opposite surface, come into contact with theheated surface of a roller. The rollers 4 typically have a surfacetemperature of about 230° C., but this will depend upon the systemparameters such as throughput speed, roller diameter, arc of contactetc. With the currently used yarn (polyester), the equipment should bearranged such that the body of the tube attains a minimum temperature of165° C., but this will vary according to the yarn used.

[0035] The knitted tube next passes round roller 5 which is attached toa load cell (not shown), and then to further driven rollers 6. The speedof rollers 6 is automatically controlled by the load cell attached toroller 5 so as to maintain a substantially constant tension—typically100 Kgs—between the rollers 3 and 5, i.e. during heating. After passingover the heated rollers 4, the tube is allowed to cool down to ambienttemperature, but whilst still under tension, as it passes over rollers 5and 6. Finally, the now-heatset tube is rolled up on a roller 7, orotherwise packed off.

[0036] If a relaxation step is required (see above) this is carried outafter the setting process and again involves passing the tube around afurther set of heated rollers at a reduced temperature, typically around170° C., and a reduced tension which could be as low as 10 Kgs,depending on the properties required. This relaxation procedure willrequire that the final output speed will be less than the output speedfrom the setting section.

[0037] During the setting operation there are a multitude of thingshappening:

[0038] the tube is being stretched due to the tension and heat;

[0039] the tube is increasing in length and reducing in width due to thestraightening of the legs of the stitches and the locking of thestitches in the machine direction;

[0040] the tube is reducing in length due to thermal shrinkage of theyarn;

[0041] the tube is increasing in strength due to a normalising of thestitches within the tube structure so that an applied load will be moreevenly spread across all of the construction yarns.

[0042] Apertures can readily be made in the tube wall by simply knittingthe yarns into an aperture. The normal knitted tube which is constructedfrom multiple rows of multiple stitches does not have any holes ofsignificant size and cannot be readily forced into significantapertures, except by cutting the yarns. These are significant advantagesover a braided tube which, as mentioned above, can readily formapertures large enough to cause the bladder to protrude.

1. A method of making an inflatable element comprising a tubular body ofknitted fabric and means for inflating the tubular body, said methodcomprising forming a tube by a knitting process using synthetic yarn,thence subjecting the knitted tube to tension and setting processes toimprove the strength and tension generating performance of the knittedtube when inflated.
 2. A method as claimed in claim 1 wherein thetension and setting processes involve the steps of heating the knittedtube whilst simultaneously maintaining the tube lengthwise undertension.
 3. A method as claimed in claim 2 further including thesubsequent step of allowing the tube to cool whilst maintaining the tubeunder lengthwise tension.
 4. A method as claimed in claim 3 wherein,after-cooling to approximately ambient temperature, the lengthwisetension is released.
 5. A method as claimed in any one of claims 2 to 4wherein the heating step comprises passing the tube over one or moreheated rollers.
 6. A method as claimed in any one of the precedingclaims wherein a relaxation stage is included in the setting process. 7.A method as claimed in claim 6 wherein said relaxation stage comprisesthe steps of heating the tube under lengthwise tension, but to a lowertemperature, and at a lower tension than the main tension and settingprocesses.
 8. A method as claimed in any one of the preceding claimswherein, after the tension and setting processes, and any optionalrelaxation stage, the tube is cut to the desired length to form one ormore of said tubular bodies.
 9. A method as claimed in any one of thepreceding claims wherein the yarn is multifilament yarn.
 10. A method asclaimed in any one of the preceding claims wherein the yarn is made fromone of the following materials: polyester, nylon, polypropylene, viscoseand HMWHDPE.
 11. A method as claimed in any one of the preceding claimswherein the knitting process for forming the tube is such as to createat least one aperture in the wall of the tube.
 12. A method as claimedin any one of the preceding claims wherein, during the knitting process,the stitch length is varied to give a particular desired shape to thefinished inflated element.
 13. An inflatable element comprising atubular body of knitted fabric and means for inflating the tubular body,said tubular body being formed from a tube made by knitting usingsynthetic yarn, followed by a tensioning and setting operation intendedto improve the strength and tension generating performance of theknitted tube during inflation.
 14. An inflatable element as claimed inclaim 13 wherein the inflating means comprises a web of flexiblematerial which is formed by impregnating the tubular body with resin.15. An inflatable element as claimed in claim 13 wherein the inflatingmeans comprises an impermeable bag or bladder which is fitted within thetubular body.
 16. An inflatable element as claimed in any one of claims13 to 15 in the 20 form of a tension tube mounted between fixed mountsso that, upon inflation, the element will be subject to an increase indiameter and a decrease in length, resulting in a build up of tension inthe element sufficient to render the element semi-rigid.
 17. Aninflatable element as claimed in any one of claims 13 to 16 25 whereinthe tubular body comprises multiple tubes which are knitted together.